Rasmussen Freja Rydahl, Kubečka Jakub, Besel Vitus, Vehkamäki Hanna, Mikkelsen Kurt V, Bilde Merete, Elm Jonas
Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus, Denmark.
Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki FI-00014, Finland.
J Phys Chem A. 2020 Jun 25;124(25):5253-5261. doi: 10.1021/acs.jpca.0c02932. Epub 2020 Jun 10.
Sampling the shallow free energy surface of hydrated atmospheric molecular clusters is a significant challenge. Using computational methods, we present an efficient approach to obtain minimum free energy structures for large hydrated clusters of atmospheric relevance. We study clusters consisting of two to four sulfuric acid (sa) molecules and hydrate them with up to five water (w) molecules. The structures of the "dry" clusters are obtained using the ABCluster program to yield a large pool of low-lying conformer minima with respect to free energy. The conformers (up to ten) lowest in free energy are then hydrated using our recently developed systematic hydrate sampling technique. Using this approach, we identify a total of 1145 unique (sa)(w) cluster structures. The cluster geometries and thermochemical parameters are calculated at the ωB97X-D/6-31++G(d,p) level of theory, at 298.15 K and 1 atm. The single-point energy of the most stable clusters is calculated using a high-level DLPNO-CCSD(T)/aug-cc-pVTZ method. Using the thermochemical data, we calculate the equilibrium hydrate distribution of the clusters under atmospheric conditions and find that the larger (sa) and (sa) clusters are significantly more hydrated than the smaller (sa) cluster or the sulfuric acid (sa) molecule. These findings indicate that more than five water molecules might be required to fully saturate the sulfuric acid clusters with water under atmospheric conditions. The presented methodology gives modelers a tool to take the effect of water explicitly into account in atmospheric particle formation models based on quantum chemistry.
对水合大气分子团簇的浅自由能表面进行采样是一项重大挑战。我们利用计算方法，提出了一种有效的途径来获取与大气相关的大型水合团簇的最小自由能结构。我们研究了由两到四个硫酸（sa）分子组成的团簇，并使其与多达五个水分子（w）水合。“干燥”团簇的结构通过ABCluster程序获得，以产生大量关于自由能的低能构象极小值。然后，使用我们最近开发的系统水合采样技术对自由能最低的构象（最多十个）进行水合。通过这种方法，我们总共识别出1145种独特的（sa）（w）团簇结构。团簇几何结构和热化学参数在ωB97X-D/6-31++G(d,p)理论水平下，于298.15 K和1 atm条件下计算得出。最稳定团簇的单点能量使用高级DLPNO-CCSD(T)/aug-cc-pVTZ方法计算。利用热化学数据，我们计算了大气条件下团簇的平衡水合分布，发现较大的（sa）和（sa）团簇比较小的（sa）团簇或硫酸（sa）分子的水合程度明显更高。这些发现表明，在大气条件下，可能需要超过五个水分子才能使硫酸团簇完全被水饱和。所提出的方法为建模人员提供了一种工具，以便在基于量子化学的大气颗粒物形成模型中明确考虑水的影响。
